The use of resistant starch (RS) is gaining increasing importance in the foodstuffs industry. Starch is mainly digested in the small intestine by the enzyme alpha amylase, which hydrolyzes the alpha-1,4-glucoside linkages of the starch to sugars. In contrast to this, resistant starch is not hydrolyzed in the small intestine by alpha amylases, but instead passes into the large intestine, where it behaves like roughage. From the degradation of RS-containing products, the body only obtains energy to a small extent. This energy input relates exclusively to the oxidative degradation of absorbed short-chain fatty acids from the large intestine. These short-chain fatty acids are end products of the carbohydrate metabolism of the intestinal microflora. With the uptake of RS-containing foodstuffs, substrates for the energy metabolism of the intestinal microflora and the large intestine epithelial cells are provided. For the maintenance of their structure and function, the latter are dependent on the luminal input of short-chain fatty acids and in particular of butyrate. Resistant starch is probably a factor for the prevention of diverticulosis and large intestine cancer.
A distinction is made between the following types of resistant starch:                RS1 Starch physically inaccessible to digestion, e.g. starch embedded in a protein or a fiber matrix. If this is broken down physically (e.g. by chewing) or chemically (e.g. by degradation of the matrix surrounding it), it can be processed by the digestive juices in the normal way.        RS2 Indigestible intact (granular) native starch granules, e.g. uncooked potato or banana starch, particularly from unripe bananas)        RS3 Indigestible retrograded starch, which is not granular        RS4 Indigestible chemically modified starch, e.g. by crosslinking or esterification (acetylation, etc.)        
In contrast to RS 4, the RS forms 1 to 3 can be made accessible to alpha amylase degradation by dissolution in NaOH or Dimethyl sulfoxide.
For the production of resistant starch, various methods have been described. Most of these methods relate to the production of RS3 starches (EP 564893 A1; EP 688872 A1; EP 846704 A1; U.S. Pat. No. 5,051,271). All these methods for the production of resistant starch comprise the dispersion and gelatinization of starch in large excess quantities of water, followed by retrogradation with the use of enzymes or acids. They are based on the view that resistant starch is formed when the amylose fraction of starch retrogrades after the gelatinization of starch. It is assumed that after gelatinization the linear amylose molecules assemble into dense double-helix configurations bound by hydrogen bridge bonds, so that the alpha-1,4-glucoside linkages are no longer accessible to alpha amylases. These methods are labor-intensive, time-consuming and can result in low yields. Furthermore, the high water content of the products can render costly drying processes necessary.
Granular starches of the RS2 type with a high content of resistant starch are mainly found in native, uncooked, wild type potato starches which depending on the estimation method display an RS content between 74-85 wt. % (Faisant et al., Sciences des Aliments 15, (1995), 83-89; Evans and Thompson, Cereal Chemistry 81(1), (2004), 31-37).
Previously known granular maize starches with high RS content are always characterized by a high amylose content (>40 wt. %). For native, i.e. granular maize starches with high amylose content, which are synthesized in various maize plants of the amylose extender (“ae”) genotype, RS values between about 40-70 wt. % were determined (Evans and Thompson, Cereal Chemistry 81(1), (2004), 31-37) by means of the RS estimation method of Englyst et al. (Europ. J. of Clinical Nutrition 46 (Suppl. 2), (1992), pp 33-50). The RS contents determined for native, i.e. granular amylomaize starch of the Hylon VII type (identical to ae VII, which was studied by Evans and Thompson) determined by Faisant et al. using two other RS estimation methods, at ca. 54 wt. % and 67 wt. % respectively, also lie in this range, which was also confirmed by a cross-laboratory study which, using different RS estimation methods, finds RS values for native amylomaize starch between about 50 and 72 wt. % (McCleary and Monaghan, J. AOAC Int. 85, (2002), 665-675). Such granular amylomaize starches from amylose extender (ae) mutants have the disadvantage of poor processing properties in certain product groups, since these starches hardly pregelatinize, and display low solubility and low swelling capacity. For applications in which only pregelatinized starches are usable or which require soluble starches or starches with swelling capacity, the amylomaize starches are thus either entirely unsuitable or they must be additionally chemically modified in order to fulfill these requirements, which is time- and cost-intensive (Senti and Russell, Tappi Vol. 43, No. 4, (April 1960), 343-349; Z. Luo et al., Starch/starch 58, (2006), 468-474).
Wheat starches with an increased RS content compared to wild type wheat plants have only recently become known and so far are only available to a very limited extent. The increased RS content of the previously known RS wheat starches is due to an increase in the amylose content, as with the amylomaize starches. In contrast to the ae mutants in maize, which are due to a mutation of the BEIIb gene from maize and have an amylose content between 50 and 90 wt. %, the increase in the amylose content necessary for raising the RS content is seen in wheat after inhibition of the gene expression of the branching enzymes IIa and IIb (Regina et al., PNAS Vol. 103 No. 10, (2006), 3546-3551). An alternative approach, which in wheat also leads to an increased amylose content and increased RS content of the wheat starch compared to the starch from wild type wheat plants, is based on the inhibition of the gene of soluble starch synthase IIa (SSIIa) (Yamamori et al., Australian Journal of Agricultural Research 57, (2006), 531-535). These SSIIa-inhibited wheat plants have a starch with an increased apparent amylose content, for which values of 37 wt. % (Yamamori et al., Australian Journal of Agricultural Research 57, (2006), 531-535) and 44 wt. % (Konik-Rose et al., Theor. Appl. Genet. 115, (2007), 1053-1065) were found. The increase in the apparent amylose content leads to an RS content of the native wheat starch of up to 3.6 wt. %, whereas native (granular) wheat starches from wild type plants contain little or no resistant starch (Yamamori et al., Australian Journal of Agricultural Research 57, (2006), 531-535). The wheat flour of these SSIIa-inhibited wheat plants leads on baking to an undesired diminution in the bread volume (Morita et al., Cereal Chemistry 79, (2002), 491-495) and the dough produced from the wheat flour displays decreased dough stability (Morita et al., Cereal Chemistry 79, (2002), 491-495; Hung et al., Cereal Chemistry 82, (2005), 690-694; Hung et al., Trends in Food Science & Technology 17, (2006), 448-456). The experts assume that an increase in the RS content of wheat starches or flours can be achieved by increasing the apparent amylose content (Morell et al., Journal of AOAC International Vol. 87 No. 3, (2004), 740-748; Yamamori et al., Australian Journal of Agricultural Research 57, (2006), 531-535).
Apart from resistant starches (RS), there is also increasing demand in foodstuffs production for starches or flours with a low content of rapidly digestible starch (rapidly digestible starch=RDS), since there is a suspicion that the continual consumption of foodstuffs with a high glycemic loading, such as for example in conventional starch-containing foodstuffs of relatively high RDS content, and the insulin release associated therewith is a risk factor in the onset of diseases such as hypertension, overweight, heart disease and type II diabetes. As a rule, foods of high RDS content have a high glycemic index (=GI) (Englyst et al., British Journal of Nutrition, 75, 327-337).
The rapid release of rather large quantities of glucose to be observed in the digestion of conventional starches/flours or of processed products from these starches/flours (e.g. bakery products and noodles) and the absorption thereof via the small intestine epithelium leads to an abrupt increase in the blood sugar level and to an outpouring of insulin (insulin response). If the RDS content of a starch or flour is decreased, then this leads to a retarded release of glucose from the starch, to a modified insulin response and hence finally to a decrease in the risk of the aforesaid diseases.
The use of wheat starches and flours with a low content of RDS appears desirable above all in those foods where the aim is a continuous release of glucose, such as for example in sports foods for endurance sports or in dietary foods to reduce the feeling of hunger.